This invention relates generally to laser cutting machines, and more particularly to laser cutting of multiple stacked worksheets.
Laser cutting machines using a focused CO.sub.2 laser beam to cut through a metal worksheet are well known in the art. Such machines are typically of fixed beam design wherein the worksheet is moved relative to the beam over a ball-transfer type work table by a carriage having suitable clamps attached to the worksheet along one edge thereof. In other arrangements, the worksheet may be clamped directly on a pin or blade-type work table which, in turn, may be moved in its entirety relative to the laser beam. Computerized numerical control (CNC) systems are conventionally provided to automatically control movement of the worksheet along the X and Y axes, as well to control movement of the laser cutting head along the Z axis toward and away from the worksheet. A suitable exhaust system may also be provided to remove gases and particulate matter generated during cutting.
The laser beam is usually focused at the upper surface of the metal sheet in order to maximize laser cutting efficiency and to achieve the cleanest and most accurate cut. (Under certain circumstances, however, it may be advantageous to focus the laser beam either slightly above or slightly under the surface.) The focused laser beam is in the form of an inverted cone, with its apex being the focal point. Since the power density of the laser beam varies inversely with its diameter, any upward movement of the worksheet into the focused beam would increase the spot diameter of the beam profile on the surface of the sheet, with a corresponding decrease in the power density. This would result in a widening of the kerf and a decrease in the concentrated heat generated. Consequently, less metal would be vaporized and more dross and slag formed.
In order to maintain a constant beam profile or laser spot diameter as the worksheet is moved relative to the beam, a sensing system may be provided which automatically adjusts the position of the laser head along the Z axis in response to irregularities or unevenness in the surface of the worksheet. In some machines the laser head is positioned above the worksheet and a so-called "spoon" sensor carried by the head contacts and rides over the surface of the worksheet. Irregularities or unevenness in the surface of the worksheet which are sensed by the "spoon" sensor are transmitted to the CNC which automatically adjusts the position of the head through a suitable control mechanism in order to maintain the proper focus. In other machines the head itself may be designed to lightly contact the surface of the worksheet by means of roller balls disposed on the underside thereof. In such arrangements, the head is constructed to have a "float" of approximately 0.75 inches so that it can freely move up and down along the Z axis against a spring bias so as to be self-adjusting.
Laser cutting of metal worksheets is typically accomplished in the presence of either reactive gases (exothermic cutting) or inert gases (endothermic cutting) which may be under several hundred PSI pressure. In exothermic cutting the reactive gases (e.g., O.sub.2, air) are ignited by the laser beam so as to reach a temperature and pressure sufficient to cut through the metal. In endothermic cutting, on the other hand, the laser beam itself cuts through the metal while the inert gases (e.g., N, He) function to blow out the slag and surround the molten edges of the sheet so as to prevent oxide formation. The latter function is particularly advantageous in so-called "clean cut" operations where it is desired to obtain weld-ready edges, free of oxides and dross, that require no expensive secondary finishing. High velocity cutting nozzles are provided to direct such cutting assist pressurized gases against the worksheet coaxial to the laser beam.
The above-described laser cutting machines have been found effective in cutting single metal worksheets of up to 0.5 inches thickness. However, attempts to cut through two or more stacked worksheets heretofore have not been successful. In such circumstances the pressurized cutting assist gases tend to flow between and separate the sheets, thus causing the top sheet(s) to move upwardly toward the laser head. This phenomenon leads to a number of undesirable results.
It has been determined in accordance with this invention that if the pressure of the cutting assist gases between stacked worksheets causes the top sheet(s) to move upwardly into the laser beam, the kerf width will increase and the cut will tend to be of poor quality with unacceptable amounts of dross formed thereon. Even on machines in which the laser head is automatically adjustable along the Z axis in response to sensed irregularities or unevenness in the surface of the worksheet, a change in the focal point in response to upward movement of the top sheet will necessarily change the distance between the focal point and the lower sheet(s). Such a change would, of course, vary the power density of the laser beam relative to the lower sheet(s), thereby adversely affecting the cut(s) therein. Moreover, if there is any separation of adjacent sheets, slag blown off of the cut through the upper sheet will tend to splatter outwardly over the lower sheet rather than being blown clearly through the cut stack as would occur if there were no separation of the sheets.
In rear-address ball-transfer machines the sheets are clamped at only one edge thereof and moved along the X and Y axes by a carriage located at the rear of the machine. Consequently, the further the cut is removed from the clamped edge, the greater is the tendency of the stacked sheets to separate. The same problem exists in other types of machines, such as pin and blade table machines. Even if the sheets are clamped to the table at more than one edge, the upper sheets may "bow-out" during cutting, particularly near the center regions of larger sheets.
It is, therefore, a primary object of this invention to provide an improved method and apparatus for laser cutting of multiple stacked metal worksheets.
More particularly, it is an object of this invention to provide a method and apparatus for laser cutting of multiple stacked metal worksheets in which pressurized cutting assist gases do not cause the top sheet(s) to move upwardly into the laser beam, thereby adversely affecting the quality of the cut.
Another object of this invention is to provide a method and apparatus for laser cutting of multiple stacked metal worksheets in which the edges of the worksheet are accurately cut and dross formation minimized.